In the past, different communications networks like public land mobile networks (PLMN), public switched telephone networks (PSTN) and data/IP networks (e.g. the public Internet) have co-existed in the form of separate monolithic networks vertically aligned with respect to each other. In each of these monolithic networks, network control and connectivity (i.e., the transfer of user data) have traditionally been bundled.
Today, mobile communications is migrating towards 3rd generation networks like the universal mobile telecommunication system (UMTS) as specified by the 3rd generation partnership project (3GPP). In parallel with the migration towards 3rd generation mobile networks, a layered network architecture that is based on horizontal planes replaces the traditional vertical network architectures. According to the horizontal approach, the tasks of network control and connectivity are being split into different horizontal planes, namely a network or call control plane on the one hand and a user plane (or connectivity plane) on the other hand. In layered communications networks, the user plane is based primarily on cell- and packet-based data transfer technologies like the asynchronous transfer mode (ATM) and the Internet protocol (IP).
In connection with the transition towards horizontally oriented network architectures, conventional components of time-division multiplexing (TDM) networks, wideband code division multiple access (WCDMA) networks, and other network components have to be adapted. In the case of TDM networks for example, mobile services switching centres (MSCs), which traditionally include network control tasks and connectivity tasks in the same node, are separated into a user plane component such as media gateway (MGW) on the one hand and a control plane component such as a dedicated server component (MSC server) on the other hand. In conventional general packet radio service (GPRS) networks a similar migration takes place. The conventional serving GPRS support node (SGSN) is split into a MGW and a dedicated server component (SGSN server).
An important task on the user plane is to provide interfaces to present-day telecommunications networks—which are typically based on TDM or (W)CDMA—and to legacy networks, such as PSTN. Accordingly, network components are required on the user plane that bridge different transmission regimes (and, if possible, add additional services like bandwidth on demand to end-user connections). As described in Magnus Fyrö et al, “Media gateway for mobile networks”, Ericsson Review no. 4, 2000, 216 to 223, MGWs are one possible realization of such bridging components. Whereas on the network control plane the MSC server controls circuit-switched (CS) services and the SGSN server controls packet-switched (PS) services, a bridging MGW on the user plane may be common to both CS and PS networks.
An the exemplary layered network architecture is shown in FIG. 1. The upper half of FIG. 1 corresponds to the network control plane including components like the MSC server or the SGSN server, whereas the lower half corresponds to the user plane including components like MGWs. In FIG. 1, fine lines represent control connections captioned with the respective control protocol, and thicker lines represent data transfer connections.
If in a scenario as depicted in FIG. 1 a call is to be set up to a mobile terminal, different network components may be involved. Usually, the network components involved are determined by the network type from which the call originates and the network type in which the call terminates. If the call originates and terminates within a particular PLMN, none of the components depicted in FIG. 1 will be involved except for the PLNM. The situation is different if the call originates from a UMTS user equipment (UE). Although the basic principles of setting up a call are similar to those conventionally employed in 2nd generation PLMN networks such as the global system for mobile communication (GSM), additional network nodes like MGWS, GGSNs, etc. will get involved.
In the exemplary scenario of FIG. 1, a call between a UMTS terrestrial radio access network (UTRAN) or a base station subsystem (BSS) and a PSTN is interconnected by two different MGWs. MGW1 for example interfaces the UTRAN and BSS, and switches ATM or routes IP traffic. The MSC server and the SGSN server both have a control connection to UTRAN and BSS. MGW2 interfaces the PSTN and is controlled using the H.248 control protocol by the MSC server and a gateway MSC (GMSC)/transit switching center (TSC) server.
FIGS. 2 to 4 schematically show the messaging involved when setting up a call from an originating UE (UE1) to a terminating UE (UE2) in a layered communications network of the type shown in FIG. 1 with a bearer independent core network (CN) as introduced by 3GPP Release 4. The bearer independent CN enables cell- and packet-based networks as the bearer, in addition to TDM bearers supported in conventional wireless networks. In general, a bearer is a transmission link with predefined characteristics (such as capacity, delay, bit error rate, etc.).
The messaging shown in FIGS. 2 to 4 reflects knowledge internal to the applicant. Any reference to the content of FIGS. 2 to 4 must therefore not be construed to acknowledge that the messaging constitutes prior art.
The call set up scenario of FIGS. 2 to 4 includes normal paging and forward bearer establishment over an ATM CN. For forward bearer establishment, bearer establishment and MGW selection are deferred. As is well known, deferred MGW selection minimizes the number of MGWs used in the call and the bandwidth used in the ATM backbone. In general, MGW selection is done using the so-called bearer-independent control (BICC) mechanism that involves signalling on the control plane.
In the following, only those messages shown in FIGS. 2 to 4 will be explained in greater detail that are required for an understanding of the framing control mechanism. Framing is a method of packing continuous user plane data into individual cells or packets. In general, an MGW supports several standardized framing formats. The Nb framing format for example has been standardized by 3GPP (see Technical Specification 3G TS 29.415). A further framing format is I.trunk, which has been standardized by the international telecommunications union (ITU). Other framing formats were standardized by the Internet engineering task force (IETF).
Referring to FIG. 2, call set up starts with UE1 contacting its associated MSC Server (MSC1) via message #1. MSC1 acknowledges the call establishment request with message #2. MSC1 then sends a call control message #3 on the control plane to the MSC Server (MSC2) associated with UE2 (as deferred MGW selection is used). Message #3 is an initial address message (IAM) that contains a placeholder “AAL2/framing”. This placeholder, however, is a dummy parameter that will not be considered further by MSC2.
With message #8, MSC2 orders Nb framing from the selected MGW associated with UE2 (i.e., from MGW2) using the parameter “3gup:interface=CN”. Likewise, with message #20, MSC1 orders Nb framing from the MGW associated with UE1 (i.e., from MGW1). Then, with message #24, Nb framing is selected and initialised for communications between MGW1 and MGW2 in ATM CN.
Nb framing is the framing format typically utilized on the user plane in connection with call control features such as forward bearer establishment/deferred MGW selection, whereas I.trunk is often the default framing format on the user plane. Due to this constellation, an implementation of forward bearer establishment/deferred MGW selection on the control plane requires the transmission of framing format instructions to the user plane. In the messaging scenario shown in FIGS. 2 to 4, this requirement is met by ordering from both MGWs the implementation of Nb framing (with messages #8 and #20 generated on the control plane).
It has been found that certain network control features such as the deferred MGW selection (including its framing-related aspects) shown in FIGS. 2 to 4 are difficult to implement under specific network conditions such as in inhomogeneous networks. Layered networks of the types shown in FIG. 1 and in the upper part of FIG. 2 may be inhomogeneous for various reasons. Network operators might for example prefer to first invest in new MSC Servers before replacing the MGWs. As a result, the new MSC servers might operate according to a new standard version, whereas the MGWs will still operate in accordance with an older version. Also, the network operator servicing UE1 in FIG. 2 might operate a newer combination of MSC Server/MGW than the network operator servicing UE2.
Accordingly, there is a need for a technique that prepares the ground for sophisticated network control features in layered communications networks. In particular, there is a need for an efficient framing selection approach that facilitates the implementation of sophisticated network control features in inhomogeneous and other networks.